The majority of your beneficial bacteria lives in the filter. You don't need to change the cartridge just swish it in some used tank water to clean it and put it back in. Did your filter also come with a little black sponge in addition to the cartridge?

OK. The sponge is for your bacteria colony and the cartridge pad is for mechanical filtration although the bacteria will live on both. But you only really need to change the pad when it's falling apart or if it's totally gunked up and impedes the water flow. Otherwise just swish it in some used tank water and put it back.